Stockers generally are installed within a semiconductor facility for temporarily storing workpieces, such as wafers, flat panel displays, LCD, photolithography reticles, or masks.
In the process of manufacturing semiconductor devices, LCD panels, and others, there are hundreds of processing equipments and thus hundreds of manufacturing steps. It is very difficult for the flow of the wafers, flat panels, or LCDs (hereafter workpiece) to be uniform from step to step, from tool to tool. Despite the best planners, there is always the unexpected scenario, such as a tool down, an emergency lot coming through, a periodic maintenance lasting longer than planned, thus there are various accumulations of the workpieces at certain steps for certain tools. The accumulated workpieces will need to be stored in a storage stocker, waiting to be processed.
Further, photolithography process is a critical process in the semiconductor fabrication facility, involving a large number of photolithography masks or reticles (hereinafter reticle). The reticles thus are typically stored in a storage stocker, and being retrieved when needed into the lithography exposure equipment.
The storage of workpieces and reticles (hereafter articles) is much more complicated due to the requirement of cleanliness. Damages to the articles can be physical damages in the form of particles, or chemical damages, in the form of interactions. With the critical dimension of the semiconductor device processing surpassing 0.1 micron, particles of 0.1 micron size, and reactive species will need to be prevented from approaching the articles. The storage areas typically would need to be even cleaner than the processing facility, to ensure less cleaning between processing.
Thus the stocker storage areas is typically designed to be sealed off from the outside environment, preferably with constant purging, and even with inert gas flow to prevent possible chemical reactions. Access to the storage areas is load-locked, to ensure isolation between the clean storage environment and the outside environment.
In a typical bare stocker system, a robot is typically used to remove the workpieces from the carrier boxes, and then loaded into a storage chamber, where the workpieces are stored without the original carrier boxes. For box stocker system, the workpieces are stored together with the carrier boxes, without the need for removing them out of the carrier boxes.
The carrier box is a protective container to minimize the substrate exposure to the environment outside of the processing machines and protect the substrate against particulate contamination. The carrier boxes are handled by an operator or by an automatic material handling system such as automatically guided or overhead transport vehicles that travel on predetermined routes, either on the ground or suspended on ceiling tracks. For semiconductor wafers, the carrier boxes are normally cassettes pods, such as SMIF (standard machine interface) or FOUP (front opening unified pod), which are handled by an operator at the tools equipment front end module (EFEM) or automatically picked up and placed in the automatic transport system.
One type of conventional transport system is an overhead transport (OHT) system, which comprises an OHT vehicle, which runs freely on a rail mounted on a ceiling. The OHT vehicle carries a cassette pod between facility equipment, such as processing systems and stockers. The OHT vehicle can load or unload a cassette pod onto a load port of the equipment, for example a MLP (Mobile Launch Platform) or an EFEM. From there, the cassette pod or the wafers can be transferred from or to the inside of the equipment.